What is the difference between sub-floor vents and floor registers/vents?

A sub floor vent refers to the ventilation of the ground floor, this maybe on a telescopic sleeve meaning the external inlet is set higher than the internal inlet.

An air register has the capability of adjusting the air flow by opening and closing a damper or flaps, usually found at the bottom of the air register. An air vent cover, simply covers the duct without the added feature of adjusting air flow. Where you have no air flow restrictor, this simply allows ventilation of fresh air to enter from different areas that will generally result in a healthier sub floor area.